Mechanical Properties and Fracture Analysis of Advanced Nickel-Based Nanomembranes

Nanoporous membranes based on the single crystalline nickel-based superalloy CMSX-4 are a promising class of materials for membranes, especially for use in premix membrane emulsification. In addition to the pore size, the strength and stability of the membrane structure are key factors for subsequent use. The production of the membranes is based on the directional coarsening of the γ/γ'-microstructure by creep deformation, in which the material is subjected to a tensile load at high temperatures so that a bicontinuous network of the γ- and γ'-phase is formed. The subsequent dissolution of the γ-phase leaves a network of γ'-phase, which can be used as a membrane structure; the former γ-matrix channels now serve as pores. Previous investigations focusing on the evolution of the microstructure during membrane fabrication found that a particularly small pore size can be achieved when the creep deformation temperature is lowered from 1000 °C to 950 °C while increasing the stress from 170 MPa to 250 MPa. This study will now investigate the strength and fracture behaviour of membranes produced by these improved parameters. For this purpose, four creep states with creep strains between 1.3% and 5.7% are investigated in tensile tests at room temperature, with the load being applied perpendicular and parallel to the raft structure. The results show that the strength of nanomembranes during perpendicular loading essentially depends on the cross-linking between γ'-rafts. Generally, an increase in creep strain leads to an increase of the cross-linking resulting in higher tensile strength. During parallel loading, γ'-inhomogeneities play an important role resulting in a loss of strength. The analysis of the fracture surfaces and evaluation of EBSD measurements reveal an insufficient cross-linking between dendrites and around γ'-inhomogeneities, leading to preferred crack paths. Therefore, the differences in orientation within the single crystal play a key role in the strength of the nanomembranes.